It is known to measure the viscosity of a fluid using a variety of types of apparatus. U.S. Pat. No. 5,565,620 discloses a method for measuring rheological properties of a fluid and a rheometer for carrying out the method. The document discloses measuring rheological properties of a sample by causing an oscillation system containing the sample to perform free oscillations. Damping caused by the sample is detected and hence viscosity can be measured. The apparatus is disclosed to be especially adapted to measure body fluids.
WO9749980 discloses an electronic viscometer having a vibrating drive in a hollow probe. The vibrating drive is configured to cause the hollow probe to oscillate in an axial direction; a feedback electrode tracks the resonant frequency of the drive. The viscometer measures viscosity of a fluid by monitoring changes in the resonant frequency of the viscometer.